Hello @Anshu kumar,
I am no longer able to recreate the issue. I have since shutdown and restarted my machine since the last time I was using the tool, so that could have something to do with it. I believe I was having the problem on both create and edit, but I don't think I edited many as I was mostly just adding new. So I am not 100% sure on that. I do know that when I did save and the app closed, it did sync because the item showed up on my phone. The crash happened immediately on the save, but it could be the save process happened quickly. The issue was not category specific. It would crash on password and login creation. Additionally I am running the UWP version of Enpass version 5.1.1 and Windows 10 Enterprise (Build 10240).
My suspicion here is that there might be something with the sync as you probably suspected as well. While the sync is successful something was happening after that crashed the app. It might have been a connectivity issue or something along those lines, but I am not certain. I will keep a watch on it and respond if it occurs again.